The present invention relates to improvements in apparatus for screening particulate material such as wood chips.
More particularly, the invention pertains to a screening deck defining a screening area, wherein the deck is formed of a series of parallel bars with spaces therebetween, with the bars uniquely arranged to increase the screen capacity through rapid orientation of the material in the direction of the slots between the bars.
In a common process for the manufacture of pulp for producing paper, logs are reduced to chips by chipping mechanisms, and the chips are cooked with chemicals at elevated pressures and temperature to remove lignin. The chipping mechanisms produce chips which vary considerably in size and shape. For the cooking process, which is known as digesting, it is desirable that the chips supplied have a uniform thickness in order to achieve optimum yield and quality; that is, to obtain a pulp which contains a low percentage of undigested and/or overtreated fibers. Under preferred conditions of digesting, the pulping chemicals or liquor penetrates into chips uniformly. If chips are provided which have too great a thickness, the liquor may not adequately penetrate the chips and the digester will produce chips with a core of under-digested fibers. If chips are provided which are too thin, the digester will produce chips that are overcooked and of low quality. To insure proper delignification of the chips in the production of pulp, the supply should not contain chips having an excessive thickness which will give rise to lack of adequate penetration during the digestion process, nor chips which are overly thin and may be overtreated during the digestion process.
Apparatus has been provided heretofore for screening chips to separate the over-thick and under-thick chips from those within the desired thickness range. Customarily, these screening devices are of the disk screen type, which have a plurality of generally circular disks mounted on parallel, rotating shafts. The disks are mounted coaxially on each shaft and spaced from each other, and the disks interleave with the disks of adjacent shafts to form screening gaps between the disks of one shaft and the disks of adjacent shafts. Through proper disk spacing, the screen can be used to separate either under-size or over-size chips from a stream of chips supplied to the screen.
One drawback associated with disk screening apparatus is that the effective or open screen area in a given screen dimension is necessarily limited, and the number of shafts provided with the disks will, therefore, be large in an industrial installation requiring substantial production capacity. Another drawback is that, by reason of precision requirements of the gaps between the disks, the manufacturing costs are relatively high. Since the disks of adjacent shafts interleave with each other in the screening area, there is friction on the surfaces interleaved due to the material to be screened becoming lodged between the disks and also by reason of resin deposits on the disks. The counter-rotational relationship between adjacent interleaved surfaces can force material into the gap, degrading chip quality and further increasing friction. It has been found that friction is one of the main causes of the high power requirements of such screen apparatus. It has also been found that it is difficult to maintain a uniform gap during operation of such apparatus, since the disks may not be mounted exactly at right angles or may become displaced slightly during operation, causing flutter with respect to each other during operation.
The disk screening apparatus heretofore used is also highly sensitive to sand, stones and scrap, and therefore subject to wear. To reduce such wear, it has been common to plate the disks with hard chromium, further increasing cost.
In my co-pending application, U.S. Ser. No. 07/629,924, I have disclosed a screening apparatus for wood chips or the like which has substantially higher industrial capacity than structures heretofore available, and which avoids the drawbacks associated with disk screening apparatus. The screen has a screening deck or bed which extends substantially horizontally, providing a large screening area. Chips are distributed across a receiving end of the screening deck, which is formed by a series of parallel bars have a unique top shape. Relative oscillatory motion is effected between sets of bars for effecting screening and moving the chips in a forward direction.
While the screen disclosed in my aforementioned co-pending application overcame many of the disadvantages of previously known screens, with high screening efficiency and greater capacity than obtainable with previously known screens, it was observed that some chips were conveyed substantial distances on the screen deck before proper presentation to a space between screen bars for the necessary gauging and screening of the chip.
It is therefore an object of the present invention to provide an improved bar screen which quickly tips and orients wood chips placed thereon for proper presentation to a screening space, to effect the necessary gauging and screening function.
It is another object of the present invention to provide a wood chip screen which has higher capacity for given screen sizes than do previously known screens of similar size.